a) Field of the invention
The present invention relates to a super-high magnification apochromatic objective lens system which has a long working distance and is to be used for inspections, etc. of IC wafers.
b) Description of the prior art
It is generally considered that the optimum magnification of a microscope as a whole lies at a level 500 to 1,000 times as high as the numerical aperture of the objective lens system used in the microscope, and the level exceeding the optimum magnification is called ineffective magnification. In order to enhance magnification of a microscope, it is therefore necessary to enlarge the numerical aperture of the objective lens system to be used therein. Though the liquid immersion type objective lens systems can have numerical apertures of 1.0 and higher, it is improper to use this type of objective lens systems in microscopes to be used for inspection of IC wafer and the objective lens systems for this application cannot have numerical apertures of 1.0 or higher.
In case of an objective lens system having a numerical aperture of 0.9, the optimum magnification of the microscope using this objective lens system lies at 450 to 900.times.. When the objective lens system has a magnification of 250.times.and the eyepiece lens system to be used in combination with it is designed for a magnification of 10.times., the microscope has a magnification of 2500.times., which is ineffective.
The description given above applies to a case where images formed by the microscope are to be observed by naked eyes. In these days, images formed by microscopes are observed not only by naked eyes but also on TV monitors and subjected to analysis. When sensitivities and resolving powers of the appliance used in conjunction with monitoring on TV and image analysis are taken into consideration, there remains a margin to allow the above-described limination imposed on the numerical aperture to be exceeded, and it is now strongly demanded to enhance magnifications of objective lens systems. Especially under the present circumstance where patterns on IC's and LSI's are becoming finer year by year, objective lens systems having high optical performance are required for inspecting highly integrated wafers.
Especially in the recent days, demands are further strengthened for objective lens systems which have super-high magnifications, apochromatic optical performance and long working distances. When objective lens systems have enhanced magnifications, however, their focal lengths are shortened, whereby chromatic aberration is aggravated and can hardly corrected in the objective lens systems. When working distances of objective lens systems are prolonged, on the other hand, it becomes very difficult to correct spherical aberration, curvature of field, coma, chromatic aberration, etc. in the objective lens systems.
As an objective lens system which has a relatively high magnification, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 60-209715. This objective lens system has a magnification of 150 to 200.times.and a mechanical working distance of 0.2 to 0.3 mm.
This conventional objective lens system has a working distance which is not long sufficiently and may allow collision against wafers.